you got me thinking! this would be really hard to make. 85mm and f1 means a HUGE front element so I don't think it will fit under the flash bulb when it is a Pancake I think it would have to look like the end of one of the big white,but then there comes the mount! I think it will look really odd/funny!back to thinking: why does lenses have to be round? every not rotating element could have its top/bottom and side parts cut of to match the sensors 3:2 format or not?wouldn't that make a 400mm 2,8 significant lighter? it would look odd, but I could life with that.

Some sort of diaphragm shutter could prevent dust from entering the camera during lens changing.

I keep wondering why they do not make a TC with zero power. That does not change magnification or degrade lens quality. Then we could leave the 'plain glass' on the camera and change lenses without fear of dust on sensor. We could just blow away, wipe away the dust from the plain glass.